1. Field of the Invention
This invention relates to a mixer circuit for use in radio communication equipment, and more particularly to an improved mixer circuit using a rat-race formed by microstrip lines.
2. Description of the Prior Art
A 3-dB power distributor is frequently used in a mixer circuit which is provided for the frequency conversion of a signal having a very high frequency. A rat-race is conveniently used to serve as this 3-dB power distributor due to the fact that the rat-race can operate stably over a wide frequency range compared with conventional branch line couplers. This rat-race has, for example, a structure as disclosed by W. H. Leighton, Jr, et al in a paper entitled "Junction Reactance and Dimensional Tolerance Effects on X-Brand 3-dB Directional Couplers" reported in IEEE TRANS-ACTION ON MICROWAVE THEORY AND TECHNIQUES, Vol. MTT-19, No. 10, Oct. 1971, pp. 818 - 824. Briefly describing this subject, the structure of this disclosed rat-race is such that a closed loop is formed by a microstrip line having a length which is three half the wavelength of a high frequency to be handled, and four microstrip lines having an impedance which is 1/.sqroot.2 times that of the microstrip line forming the closed loop are connected to the closed loop at intervals of quarter wavelengths starting from any desired point on the closed loop. This rat-race has such a property that, when a signal is applied by way of the first of these four lines, one half of the supplied power is transmitted to each of the second and fourth lines, while no signal is transmitted to the third line.
FIGS. 1 and 2 show principal parts of mixer circuits utilizing such a rat race. In the mixer circuit shown in FIG. 1, microstrip lines 1, 2, 3, 4 and 5 constitute the rat-race. In the form shown in FIG. 1, the lines 3 and 5 have the same length and are connected at one end thereof to the line 1 and at the other end thereof to one terminal of the diodes 6 and 7 respectively. Low-pass filters 8 and 9 are connected to the other terminal of the diodes 6 and 7 respectively for deriving intermediate frequency signals from the diodes 6 and 7. These diodes 6 and 7 are disposed in directions opposite to each other so that intermediate frequency signals of the same phase can be obtained by rotating the phase of a high frequency signal input by the rat-race. In order that the intermediate frequency signals can be derived from the low-pass filters 8 and 9, the impedance of the rat-race when viewed from the diodes 6 and 7 must be shorted against the frequency of the intermediate frequency signals. Therefore, when the lines 4 and 2 are selected as a high frequency signal input line and a local oscillation signal input line respectively, it is necessary to connect to the line 2 a stub filter 10 which is open-circuited against the local oscillation frequency and short circuited against the intermediate frequency. The mixer circuit of the structure shown in FIG. 1 is however defective in that a signal combining circuit (not shown) for combining the intermediate frequency signal outputs of the low-pass filters 8 and 9 must be disposed to extend across the high frequency signal input line 4 in the space above the plane of the mixer circuit or such signal combining circuit must be formed by extending a lead wire to the area outside of the mixer circuit.
In the mixer circuit shown in FIG. 2, microstrip lines 1, 2, 3, 4 and 5 constitute the rat-race similarly, and diodes 6 and 7 are similarly connected at one terminal thereof to one end of the lines 3 and 5 respectively. In the form shown in FIG. 2, the diodes 6 and 7 are shorted to ground at the other terminal thereof, and a low-pass filter 13 is connected to the local oscillation signal input line 2 in order to avoid an undesirable power loss of a high frequency signal input applied by the line 4. In order that an intermediate frequency signal can be derived through this output filter 13, the circuit portions except the filter 13 must be open-circuited against the intermediate frequency when viewed from the diodes 6 and 7. Therefore, capacitive means 11 and 12 which are short-circuited against a high frequency and open-circuited against a low frequency are connected in series with the input lines 2 and 4 respectively. Such capacitive means 11 and 12 are formed by cutting off a portion of suitable length from the respective lines 2 and 4. In the mixer circuit of the structure shown in FIG. 2, the intermediate frequency signals appearing from the diodes 6 and 7 are combined by the output filter 13 in the state in which a phase difference exists therebetween, and an undesirable reduction in the power of the output signal results due to the fact that the distance between the diode 6 and the output filter 13 differs from that between the diode 7 and the output filter 13. This adverse effect becomes more marked with the increase in the frequency of the intermediate frequency signal. Further, due to the fact that the output filter 13 is connected to the line 2 to which the local oscillation signal input of large power is applied, it is necessary to provide a plurality of filters in order to prevent appearance of the local oscillation frequency at the output terminal due to leakage.
The mixer circuits using the rat-race have such advantage that the local oscillation signal of large power which may be reflected by the diodes does not appear in the high frequency signal input line, in addition to such inherent advantage of the rat-race that it can operate stably over a wide frequency range. However, the mixer circuit structures above described are defective in that the intermediate frequency signal output means cannot be disposed in the same plane as that of the rat-race and an external circuit must be additionally provided, or the output power loss increases with the increase in the frequency of the intermediate frequency signal.